The present invention relates to a photomask for use in fine pattern formation in the fabrication of a semiconductor integrated circuit device, a pattern formation method using the photomask and a method for creating mask data for the photomask.
Recently, there are increasing demands for thinning of circuit patterns in order to further increase the degree of integration of a large scale integrated circuit device (hereinafter referred to as the LSI) realized by using semiconductors. Accordingly, thinning of an interconnect pattern used in a circuit or thinning of a contact hole pattern (hereinafter referred to as the contact pattern) for mutually connecting multilayered interconnects having an insulating layer therebetween has become very significant.
Now, the thinning of an interconnect pattern using a conventional optical exposure system will be described on the assumption that the positive resist process is employed. In the positive resist process, a line pattern corresponds to a line-shaped resist film (a resist pattern) remaining correspondingly to an unexposed region of a resist after exposure using a photomask and subsequent development. Also, a space pattern corresponds to a resist removal portion (a resist removal pattern) corresponding to an exposed region of the resist. Furthermore, a contact pattern corresponds to a hole-shaped resist removal portion and can be regarded as a particularly fine space pattern. It is noted that when the negative resist process is employed instead of the positive resist process, the above-described definitions of a line pattern and a space pattern are replaced with each other.
In general, a fine pattern formation method using oblique incident exposure (off-axis illumination) designated as super-resolution exposure has been introduced for the thinning of an interconnect pattern. This method is good for thinning a resist pattern corresponding to an unexposed region of a resist and has an effect to improve the depth of focus of dense patterns periodically and densely arranged. However, this oblique incident exposure has substantially no effect to thin an isolated resist removal portion, and on the contrary, the contrast and the depth of focus of an image (optical image) are degraded when it is employed for an isolated resist removal portion. Therefore, the oblique incident exposure has been positively employed in pattern formation in which a resist removal portion has a larger dimension than a resist pattern, such as gate pattern formation.
On the other hand, it is known that a small light source including no oblique incident component and having a low degree of coherence is effectively used for forming an isolated fine resist removal portion like a fine contact pattern. In this case, the pattern can be more effectively formed by using an attenuated phase-shifting mask. In an attenuated phase-shifting mask, a phase shifter is used instead of a completely shielding portion as a shielding pattern for surrounding a transparent portion (an opening) corresponding to a contact pattern. The phase shifter has very low transmittance of approximately 3 through 6% against exposing light, and causes phase inversion of 180 degrees in the exposing light with respect to light passing through the opening.
Herein, the transmittance is effective transmittance obtained by assuming that a transparent substrate has transmittance of 100% unless otherwise mentioned. Also, a completely shielding film (a completely shielding portion) means a shielding film (a shielding portion) having effective transmittance lower than 1%.
Now, the principle of a conventional pattern formation method using an attenuated phase-shifting mask will be described with reference to FIGS. 32A through 32G.
FIG. 32A is a plan view of a photomask in which an opening corresponding to a contact pattern is formed in a chromium film formed on the mask surface as a completely shielding portion, and FIG. 32B shows the amplitude intensity obtained in a position corresponding to line AA′ of light having passed through the photomask of FIG. 32A. FIG. 32C is a plan view of a photomask in which a chromium film corresponding to a contact pattern is formed in a phase shifter formed on the mask surface, and FIG. 32D shows the amplitude intensity obtained in a position corresponding to line AA′ of light having passed through the photomask of FIG. 32C. FIG. 32E is a plan view of a photomask in which an opening corresponding to a contact pattern is formed in a phase shifter formed on the mask surface (namely, an attenuated phase-shifting mask), and FIGS. 32F and 32G respectively show the amplitude intensity and the light intensity obtained in a position corresponding to line AA′ of light having passed through the photomask of FIG. 32E.
As shown in FIGS. 32B, 32D and 32F, the amplitude intensity of the light having passed through the attenuated phase-shifting mask of FIG. 32E is a sum of the amplitude intensities of the lights having passed through the photomasks of FIGS. 32A and 32C. In other words, in the attenuated phase-shifting mask of FIG. 32E, the phase shifter working as a shielding portion is formed not only so as to transmit light at low transmittance but also so as to cause an optical path difference (a phase difference) of 180 degrees in the light passing through the opening with respect to the light passing through the phase shifter. Therefore, as shown in FIGS. 32B and 32D, the light passing through the phase shifter has an amplitude intensity distribution in the opposite phase with respect to the light passing through the opening. Accordingly, when the amplitude intensity distribution of FIG. 32B and the amplitude intensity distribution of FIG. 32D are synthesized with each other, a phase boundary having amplitude intensity of 0 (zero) is obtained as a result of the phase change as shown in FIG. 32F. As a result, as shown in FIG. 32G, at an end of the opening corresponding to the phase boundary (hereinafter referred to as the phase end), the light intensity, which is expressed as a square of the amplitude intensity, is 0 (zero), and thus, a strongly dark part is formed. Therefore, in an image of the light having passed through the attenuated phase-shifting mask of FIG. 32E, very high contrast can be realized around the opening. However, this improved contrast is obtained in light vertically entering the mask, and more specifically, light entering the mask from a small light source region with a low degree of coherence. On the other hand, such improved contrast cannot be obtained even around the opening (namely, in the vicinity of the phase boundary where the phase change is caused) in employing the oblique incident exposure, such as exposure designated as annular illumination excluding a vertical incident component (an illumination component entering from the center of the light source (along the normal direction of the mask)). Furthermore, as compared with the case where the exposure is performed by using small light source with a low degree of coherence, the depth of focus is disadvantageously smaller when the oblique incident exposure is employed.
Moreover, in order to compensate the disadvantage of the attenuated phase-shifting mask in the oblique incident exposure such as the annular illumination, a method in which a small opening that is not resolved, namely, an auxiliary pattern, is formed around an opening (corresponding to an isolated contact pattern) of the attenuated phase-shifting mask has been proposed (for example, see Japanese Laid-Open Patent Publication No. 5-165194). Thus, a periodic light intensity distribution can be obtained, thereby improving the depth of focus.
As described above, in the case where a fine resist removal pattern such as a contact pattern is to be formed by the positive resist process, it is necessary to perform the exposure by using a combination of an attenuated phase-shifting mask and a small light source with a degree of coherence of approximately 0.5 or less, that is, illumination having a vertical incident component alone. This method is very effective for forming a fine isolated contact pattern.
In accordance with recent increase of the degree of integration of semiconductor devices, it has become necessary to form, as not only interconnect patterns but also contact patterns, isolated patterns and patterns densely arranged at a pitch corresponding to the wavelength. In such a case, in order to realize a large depth of focus in the formation of densely arranged contact patterns, the oblique incident exposure is effectively employed as in the formation of densely arranged interconnect patterns.
In other words, the oblique incident exposure is indispensable for the formation of dense interconnect patterns and dense contact patterns, but when the oblique incident exposure is employed, the contrast and the depth of focus of isolated contact patterns and isolated space patterns between interconnects are largely degraded. This degradation of the contrast and the depth of focus is more serious when an attenuated phase-shifting mask is used for improving the resolution.
On the contrary, when a small light source with a low degree of coherence is used for forming isolated fine contact patterns and isolated fine space patterns between interconnects, it is disadvantageously difficult to form dense patterns and fine line patterns.
Accordingly, there is a reciprocal relationship between the optimum illumination conditions for isolated fine space patterns and the optimum illumination conditions for densely arranged patterns or fine line patterns. Therefore, in order to simultaneously form fine resist patterns and fine isolated resist removal patterns, trade-off is considered between the effect of a vertical incident component and the effect of an oblique incident component of the light source. As a result, a light source with an intermediate degree of coherence (of approximately 0.5 through 0.6) is used. In this case, however, both the effects of the vertical incident component and the oblique incident component are cancelled, and therefore, it is difficult to realize a higher degree of integration of semiconductor devices by simultaneously thinning isolated line patterns or dense patterns and isolated space patterns.
It is noted that the aforementioned auxiliary pattern needs to be provided in a position away from an opening corresponding to a contact pattern at least by a distance corresponding to the wavelength of a light source (exposing light). Therefore, in the case where openings are arranged at a pitch ranging from the wavelength to a double of the wavelength, the auxiliary pattern cannot be used. In other words, the aforementioned method using the auxiliary pattern is not applicable to all arrangements ranging from the case where openings are arranged at a pitch substantially corresponding to the wavelength to the case where an opening is isolated.